Article

Punch Tip and Head Pressures

My August 2010 column provided incorrect formulas for calculating punch-tip cutting pressures. The proper explanations and formulas should have appeared as follows:

The punching force is the product of the punch tip profile length (L) times the sheetmetal thickness (t) times the sheetmetal’s shear strength (st), or:

Fp = (L)(t)(st)

The shear strength for mild steel is approximately 70 percent of its ultimate tensile strength, aluminum is about 50 percent and stainless steel is about 90 percent. Shear strengths also can vary significantly within the same material type. For example, the shear strengths for copper alloys have been reported to be between 50 and 90 percent of their ultimate tensile strength, depending on the alloy.

After punching force (Fp) is known, the tip pressure (Pt) can be calculated. For a standard shoulder punch, Pt is equal to the punching force divided by the cross-sectional area (A) of the punch tip:

Pt = Fp / [(p)(½d)2]

If the punch has a spring elector pin, the

Fig. 1

cross-sectional area is reduced by the area of the hole in the punch face. The force on an ejector type punch, similar to that shown in Fig. 1, is found by:

Pt = Fp / [(p)(½d)2 – (½d1)2]

For those that tried to use the formula from my August column—only to find yourselves thoroughly frustrated—I apologize for the error.

I received several e-mails from readers who were not having problems with punch-tip breakage, but instead were experiencing punch-head breakage. To solve punch-head breakage problems, we must first understand why punch heads break.

Causes for Breakage

There are two main causes for head breakage: high impact force and high snapthrough forces. Impact failure results from excessive loading, which literally crushes the head. This type of failure usually occurs when the cutting clearance is tight and/or the part material is hard or thick. Conversely, snapthrough failures occur when there is a sudden unloading of pressure on the punch. This phenomenon is associated with increased punch-to-die clearance and with high-strength materials.

Other factors that contribute to head breakage are punch pumping, due to poor fitup of the punch body to the punch holder, and high-hardness backing plates.

Solutions

Reducing the head diameter or chamfering the contact surface on the head (Fig. 2)

Fig. 2

will minimize compressive loading and flex at the unsupported outside diameter of the head at initial punch impact.

Applying a shear angle to the punch point will help reduce the forces on the punch body and the punch head.

Reducing the hardness of the backing plate is another viable solution. The backing plate must adsorb the punch impact adequately without cracking, breaking, permanently deforming or severely deflecting. Softer and tougher tool steels such as S-7 are good candidates for backing plates.

When all else fails, using a punch with a larger body usually will reduce head breakage.

Proper fitup between the punch body and punch holder is essential to prevent punch pumping, which basically acts as a battering ram on the punch head. Good design and build practices should include the following (see Fig. 3):

Fig. 3

• Make the body of the punch a slight press fit into the hole in the retainer. A press fit between 0.0003 to 0.0005 in. should be sufficient.

• Ensure that the head is fully supported and seated into the counterbored hole in the retainer.

• Ensure that the retainer is cleared for the radius connecting the head to the shank. Provide just enough clearance for the radius. Avoid excessive clearance.

• Ensure the head of the punch does not protrude past the bottom surface of the retainer.

• Fit the punch so that the bottom of the head is as close as possible to the bottom surface of the retainer, but no more than 0.001 in. below.